1. Field of the Invention
This invention relates to circuit protection devices comprising PTC conductive polymers.
2. Introduction to the Invention
Conductive polymer compositions exhibiting PTC behavior, and electrical devices comprising them, are well known.
Particularly useful devices comprising PTC conductive polymers are circuit protection devices. Such devices have a relatively low resistance under the normal operating conditions of the circuit, but are "tripped", i.e., converted into a high resistance state, when a fault condition, e.g., excessive current or temperature, occurs. When the device is tripped by excessive current, the current passing through the PTC element causes it to self-heat to an elevated temperature at which it is in a high resistance state. The increase in resistance is accompanied by an expansion of the PTC element along an expansion axis. Such devices, and PTC conductive polymer compositions for use in them, are described for example in U.S. Pat. Nos. 4,237,411, 4,238,812; 4,255,698; 4,315,237; 4,317,027; 4,329,726; 4,352,083; 4,413,301; 4,475,138; and 4,481,498 and in copending commonly assigned U.S. application Ser. Nos. 141,989 and 628,945 and in the commonly assigned patent applications filed contemporaneously with this application by Deep et al, Ser. No. 711,909, by Carlomagno, Ser. No. 711,790, by Au et al, Ser. No. 711,910 and by me, Ser. No. 711,908. The disclosure of each of these patents and pending applications is incorporated herein by reference.